1. Field of the Invention
The present invention relates to an optical modulator equipped with an optical modulation element consisting of an electro-optic crystal, the optical modulator being employed in recorders such as laser printers, digital copying machines and the like and primarily being employed in recorders in the printing field which express gradation with mesh points onto a binary sensitive material, particularly a heat-mode sensitive material.
2. Description of the Related Art
Recorders, such as laser printers, digital copying machines and the like, which perform the recording of an image onto a heat-mode sensitive material with laser light or the like have hitherto been utilized.
When recording is performed on a heat-mode sensitive material with laser light, there is a need to employ laser light with considerably greater power. At the incident surface of an optical modulation element equipped inside of the recorder, the power density sometimes reaches as much as a few tens of kW/cm.sup.2. If laser light is modulated with an optical modulation element consisting of an electro-optic crystal as the optical modulation element of the aforementioned recorder for recording an image on a heat-mode sensitive material, generally the electro-optic crystal will absorb light and the electro-optic element will generate heat, although there are a few exceptions. This generation of heat causes the electro-optic element to rise in temperature. Due to this temperature rise, if the element temperature exceeds a Curie point, the electro-optical effect will be considerably reduced and the electro-optic element will not be able to be used as an optical modulation element. There are some cases where the electro-optic element is destroyed.
In recorders such as laser printers, digital copying machines and the like, an optical modulation element array consisting of an electro-optic crystal is generally used. However, in recorders for recording an image on a heat-mode sensitive material, it is not normal practice to use an electro-optic element consisting of an electro-optic crystal.
Japanese Unexamined Patent Publication No. 4(1992)-49065 discloses an optical modulator employing an electro-optic crystal, in which the temperature of the electro-optic crystal is adjusted. In this publication it has been described that means for monitoring the temperature of an electro-optic element and heating the element is provided, in view of the problem that the operating characteristics of the element vary with ambient temperature and therefore the element cannot be used until the temperature within the optical modulator reaches a steady state. Also, Japanese Unexamined Patent Publication No. 4(1992)-161922 discloses an optical modulator employing a lead lanthanum zirconate titanate (PLZT) element as an electro-optic crystal. The optical modulator is provided with temperature detection means and heating means in order to avoid the hysteresis in an electric-field/transmitted-light-quantity operation which occurs near the PLZT element at room temperature, by heating up the PLZT element to the vicinity of a Curie point.
In the aforementioned conventional two techniques, however, no consideration has been given to the case where the respective optical modulators are employed during the recording of an image onto a heat-mode sensitive material. Therefore, when light recording is performed on a heat-mode sensitive material by an optical modulator with similar construction, there is a need to employ laser light having high power, as described above. As a result, a problem will arise in that the generation of heat in the electro-optic crystal will cause nonconformity in the element itself or light modulation.
On the other hand, Japanese Unexamined Patent Publication No. 4(1992)-175721 discloses an optical modulator in which an optical modulation element consisting of an electro-optic crystal is fixed to a base plate having nearly the same linear expansion coefficient as the linear expansion coefficient of the optical modulation element.
However, in the case where light recording is performed on a heat-mode sensitive material by an optical modulator constructed so that the linear expansion coefficient of an electro-optic crystal substrate is matched with that of a base plate, the degree of expansion of the electro-optic element becomes greater than that of the base plate (fixing member), because generally the temperature of the electro-optic element becomes higher than that of the base plate. Therefore, if the expansion coefficients of the base plate and the electro-optic crystal substrate are made equal to each other, a large amount of warping will occur in the electro-optic crystal element, and the warped element cannot be used as an optical modulation element because the electro-optic characteristics vary due to the warp. This warping results in the breakdown of the element in certain circumstances.